1. Field of the Invention
The present invention relates to a surface emitting laser, a surface emitting laser array, and an image forming apparatus using the surface emitting laser array.
2. Description of the Related Art
A vertical cavity surface emitting laser (VCSEL) is a laser for emitting a laser beam in a direction perpendicular to the in-plane direction of a semiconductor substrate.
A Distributed Bragg Reflector (DBR) is normally used as a reflecting layer of the surface emitting laser. The DBR is generally formed by alternately stacking a high-refractive index layer and a low-refractive index layer at an optical film thickness of λ/4.
The surface emitting laser has excellent characteristics in which a stable single mode is obtained as a longitudinal mode, a threshold value thereof is lower than a threshold value of an edge emitting laser, and a two-dimensional array is easily formed. Therefore, applications of the surface emitting laser are expected as a light source for optical communication and light transmission and a light source for electrophotography.
It is important for the surface emitting laser to control a lateral mode of oscillation. When the surface emitting laser is applied to optical communications, a desirable lateral mode output is the single mode. Therefore, in the case of the surface emitting laser, a current confinement structure is provided by selective oxidation in an inner portion of the laser to limit a light emitting region of an active layer. Simultaneously, a wave guiding structure is formed on a selectively oxidized portion for the single lateral mode.
However, when a single lateral mode oscillation is to be performed by only this method, it is necessary to reduce an oxide confinement diameter. When the oxidation confinement diameter reduces, the light emitting region becomes narrower, so it is difficult to obtain a large laser output power.
Therefore, according to Japanese Patent Application Laid-Open No. 2004-063657, there is proposed a structure in which a fundamental mode satisfies a resonance condition and a high-order mode does not satisfy the resonance condition, thereby realizing the single lateral mode at high output power.
FIG. 8 illustrates a surface emitting laser described in Japanese Patent Application Laid-Open No. 2004-063657. A bottom DBR 1020 made of a semiconductor is formed on a substrate 1010. A clad layer 1030 is formed on the bottom DBR 1020. The clad layer 1030 includes an oxide confinement structure which has an active layer 1035, an oxidized region 1040, and a non-oxidized region 1050. An etching stop layer 1060 and a contact layer 1070 are formed on the clad layer 1030. A top DBR 1080 made of a dielectric is provided to a central region of a mesa structure.
The surface emitting laser has characteristics in which a resonator length between the top DBR 1080 and the bottom DBR 1020 in the central region is different from a resonator length therebetween in a peripheral region. That is, a thickness 1090 of the central region which is a fundamental mode selection region is Nλ/2n (N: integer, λ: oscillation wavelength, n: effective refractive index in medium). In contrast, a thickness 1100 of the peripheral region which is a high-order mode oscillation suppression region is Nλ/2n+λ/4n.
As described above, when the layer thickness is changed between the fundamental mode selection region and the high-order mode oscillation suppression region, only the fundamental mode selection region satisfies the resonance condition, with the result that the single mode oscillation is obtained.
In the structure of the laser disclosed in Japanese Patent Application Laid-Open No. 2004-063657 as described above, the physical thickness is changed between the fundamental mode selection region and the high-order mode oscillation suppression region and the top DBR 1080 is formed on such regions.
However, according to the structure disclosed in Japanese Patent Application Laid-Open No. 2004-063657, the fundamental mode selection region and the high-order mode oscillation suppression region are different from each other in film thickness by λ/4n, so it is difficult to stack the top DBR 1080 on such regions. That is, because the top DBR 1080 is formed on such an uneven surface region, it is difficult to make the film thickness of the DBR uniform. As a result, it is difficult to obtain a desired reflectance.
In particular, it is difficult to form, on the uneven surface region, a film made of a semiconductor material to which a current is easily injected, to produce the DBR.